Field-oriented motor control has become a common technology in the industry for achieving reduced electricity consumption and vibration. A sensor-less field oriented control, a variation of field-oriented control is increasing its application in meeting such requirements. A sensor-less field-oriented control drives the motor by estimating the rotational speed and position of the rotor (based on feed back control) and not by detection of the position sensor such as hall IC.
Applications requiring high-speed motor rotation employ a control method called over-modulation control. The over-modulation control modulates the modulation factor of sinusoidal applied voltage to be greater than 1.0 to increase effective output voltage and to reduce output current, which in turn reduces copper-loss and consequently, electricity consumption. Such control method is disclosed, for example in JP 2005-160183 A.
A combined use of sensor-less field oriented control and over-modulation control gives rise to the following problem. In over-modulation control, since the output voltage takes a distorted sinusoidal waveform, a harmonic component occurs in the output voltage and further in the output current. On the other hand, in sensor-less field-oriented control, since the above mentioned output voltage and output current are considered for calculation of rotational position of the rotor, if the rotational position is estimated during execution of over-modulation control, the result of estimation is affected by the harmonic component.
Further, since motor speed control and current control are executed based on the above mentioned estimation result, the harmonic component is circulated in the control system, consequently destabilizing the control and in some cases, possibly stopping the rotation of the motor.